The present invention relates to methods and apparatus for producing carbon dioxide snow and more particularly, to methods and apparatus for depositing a substantially uniform blanket of CO.sub.2 snow over a relatively large surface area.
Carbon dioxide has been extensively used as a refrigerant for both feezing and chilling items of food such as red meat, poultry, vegetables, etc. It is well known to utilize carbon dioxide in the solid phase as a refrigerant and to form solid CO.sub.2, i.e. snow, by expanding a flow of liquid carbon dioxide through a nozzle with the resulting drop in pressure effecting a phase change to solid and gaseous CO.sub.2. The solid CO.sub.2 or snow will exhibit a temperature of -109.degree. F. and may be directed downwardly upon items to be frozen or chilled while the resulting CO.sub.2 vapor is exhausted by means of a suction blower or other suitable device.
In order to control streams of CO.sub.2 snow, devices commonly known as snow horns have been developed. In these devices, a nozzle is disposed so as to emit a stream of solid-gaseous CO.sub.2 into a cylindrical horn section having one end thereof closed while the other end is open so as to enable the solid-gaseous CO.sub.2 stream to be emitted therefrom in a particular direction. Generally, such snow horn devices are mounted vertically with the bottom end being open so that solid CO.sub.2 may be deposited under gravity onto items to be refrigerated or chilled. For example, it is known to utilize a pair of snow horns, each adapted to receive a supply of liquid CO.sub.2 to deposit solid CO.sub.2 onto items to be refrigerated which are contained in a carton or other similar sized container. Typically, food products are chilled in this manner, and upon sealing the container, food products may be shipped as a "fresh" food product which is maintained at a temperature slightly above the freezing point thereof by virtue of refrigeration supplied by the CO.sub.2 snow. It has been found that these types of chilling systems are effective in supplying carbon dioxide throughout relatively small cartons, i.e. 18".times.28" and about 10" deep. Typically, the pair of snow horns utilized in this system are enclosed in a hood which is adapted to enable the resulting CO.sub.2 vapors to be exhausted from the area immediately surrounding the container.
Recently, food processors have found it economical to transport and/or store one or more food products such as red meat, poultry, etc. in a relatively large container commonly referred to as a "combo". These containers are typically about 42".times.50" and about 30"-36" deep. It is known to attempt to refrigerate food products such as meat, etc. introduced into these larger containers or combos by concurrently directing CO.sub.2 snow from as many as 4-6 snow horns into the container in an effort to uniformly refrigerate the interior thereof. It is important to provide such refrigeration so as to avoid "hot spots" from developing in the container and particularly in corners thereof as bacteria will form almost immediately in these areas and consequently the quality of the food products being transported and/or stored therein is rapidly degraded. Attempts to avoid the occurrence of hot spots include the manual positioning of one or more snow horns to assure that solid carbon dioxide supplied to such containers actually reaches its corners. However, this technique is relatively inefficient and incurs significant labor costs and is not always accurate in assuring that such hot sots do not develop as mentioned above.
In U.S. Pat. No. 4,415,346, there is described a snow horn device wherein liquid CO.sub.2 is emitted through an orifice at the upper end of a horn member and impinges interior walls of the horn device. The resulting stream of CO.sub.2 solid is deflected downwardly so that it exits the horn device and CO.sub.2 vapor is exhausted from the bottom of the horn device into an enclosure surrounding the horn so that the vapor may be removed from the area onto which CO.sub.2 snow is to be deposited. The resulting accumulation of CO.sub.2 snow, as illustrated FIG. 5 of this patent, is a mound and relatively little CO.sub.2 snow will accumulate in the corners of the container therefor. Consequently, the apparatus illustrated in this reference is not considered to be effective to generate a substantially uniform blanket of CO.sub.2 snow over a relatively large surface area. A further example of apparatus for forming carbon dioxide snow is illustrated in U.S. Pat. No. 4,287,719. In this apparatus, a J-tube is utilized to form a stream of solid CO.sub.2 by separating the solid and gaseous phases of a mixed phase stream and in turn directing the substantially solid CO.sub.2 flow into a pair of circular baffles to establish a substantially spiral flow therein. As the streams exit the lower extremity of the circular baffles, the streams interact so as to form larger sized carbon dioxide particles which then descend under gravity downwardly and exit from a larger enclosure surrounding the circular baffles. This reference is primarily concerned with exhausting CO.sub.2 vapor to the greatest extent possible and is not concerned with the problems associated with forming uniform blankets of solid carbon dioxide.
Accordingly, in order to adequately refrigerate relatively large containers for products such as food, etc., there is a clear requirement for methods and apparatus for forming substantially uniform blankets of solid carbon dioxide throughout such containers. More particularly, there is a need for methods and apparatus for effectively converting a point source discharge of solid/gas carbon dioxide into an essentially rectangular configuration but in such a manner that the resulting pattern of solid carbon dioxide exists as a blanket that extends uniformly over the desired rectangular area. It is to this end that the present invention is directed as will be seen from the subsequent detailed description thereof.